Food, confectionery, general merchandise for everyday use, pet food, etc., adopt packaging materials that use various types of plastic films, to take advantage of their design property, economy, content protection property, ease of transportation, and other functions. Also, many packaging materials are printed on by means of gravure printing or flexographic printing, with the intent of adding design or message features that would appeal to consumers.
And, to obtain the intended packaging material, front printing is performed which involves printing on the front side of the base film of the packaging material, or an adhesive or anchor agent is applied on the printing side of the base film of the packaging material, as necessary, after which reverse printing is preformed to laminate the film.
In reverse printing, colored inks and white ink are printed one by one on a polyester or nylon film, aluminum foil, or any other type of film, and then a polyethylene film, polypropylene film, or the like, is layered on the white-ink printed layer, for the purpose of heat sealing, by means of dry lamination using adhesive, extrusion lamination using anchor coating agent, etc. (refer to Patent Literature 1, for example).
Thereafter, the laminate is used as a laminate bag for storing food, such as confectionery, soup, miso soup, or the like. Laminate bags like these are opened by human hands in order to take out the contents. Here, the bags must be easy to tear because, if they are difficult to tear, excessive force must be applied to open the bag, or the bag may tear in an unintended direction, thereby causing the contents to spill.
In particular, areas that are printed with ink tend to have poor tearing property because the binding force between the ink layer and adhesive layer is lower compared to non-printed areas.
A known means for improving the tearing property is to make the laminate adhesive layer harder (refer to Patent Literature 2, for example); if ink areas are interspersed with non-printed areas without ink, however, a problem occurs that adjusting the hardness of the adhesive layer to a level that permits tearing of the ink areas would make the non-printed areas too hard, while using the hardness of the non-printed areas as a reference would result in poor tearing property of the ink areas.
To solve this problem, an art of blending into an ink layer a curing agent that contains two or more functional groups that can react with the same functional groups in the ink layer and adhesive layer, is proposed, wherein the ink layer is formed according to a composition constituted by a compound (a1) that contains two or more hydroxyl groups, as functional groups, in its molecule, and also by a curing agent that reacts with these hydroxyl groups (refer to Patent Literature 3, for example). In the examples cited, however, the ratio of the solid content of the polyfunctional isocyanate curing agent, relative to that of the polyurethane resin, in the ink layer, is very low at 1:0.25 (polyurethane resin:polyfunctional isocyanate curing agent), which presents a problem that, although the tearing property improves slightly, it is still not sufficient and resistance is felt while tearing.
Also, ink layers using such compound do not develop color fully.